Cathode ray tube



Dec. 2, 1941. l.. F. BROADWAY CATHODE RAY TUBE Filed April 4, 1940 ami yw f w WN m INVEN TOR. LEONARD FRANC/5 BROADWAY ATTORNEY.

Patented Dec. 2, 1941 UNITEj 'Si' TENT OFFICE CATHODE RAY TUBE LeonardFrancis Broadway, YIclfienharn, England,

assigner to Electric & Musical Industries Limited, Hayes, Middlesex,England, a company of Great Britain Application April 4, 1940, SerialNo. 327,763

In Great Britain April 4, 1939 2 Claims.

'lihisinvention relates to cathode ray tubes in .which the electron beamis focussed by electromagnetic means.

:that the Yoscillation is due to the interaction of the electron beamAin the presence of a magnetic field, with a beam -of positive ionswhich are formed by the ionisation of the residual gas in the tube byelectron impact, the positive ions being caused to move towards thecathode by the same electrostatic field which accelerates the electronsAaway from the cathode. It has also been -found that in order that theline distortion referred to shall be produced, the positive ion beammust be concentrated or focussed to some extent near the region in whichthe paths of the electrons cross and at which the oscillation ispresumed to occur.

As the effect referred to is dependent upon the formation of ions withinthe tube, it will be clear that the distortion could be eliminated byproducing such a high vacuum in the tube that substantially noionisation occurs. The production of the degree of Vacuum which would benecessitated is not easy in practice and it is the object of the presentinvention therefore to provide a simple and eicacious method of removingthe distortion without depending on the maintenance of an exceedinglyhigh degree of vacuum within the cathode ray tube.

In arriving at the solution upon which the invention is based, it hasbeen assumed that the focussing or concentration of the positive ionbeam is inuenced to a considerable extent by the magnetic field set upby a coil provided to focus the electron beam. Although the positiveions are much less susceptible to magnetic concentration by reason ofthe relative smallness of their charge/mass ratio, yet in cathode raytubes as constructed at the present time, the magnetic focussing iseffective in a region of substantially zero electrostatic field in whichthe ions have a very low velocity.

According to the present invention, in a method of reducing oreliminating distortion of the scan- This line distortion which resultsin ning ,lines in a cathode ray tube'due to thecause mentioned, thevelocity of the positive ions inthe neighbourhood of the magneticfocussing coil is caused to increase toa degree such ythat the magneticfield dueto Vthe coil 'is incapable of producing sufficientconcentration .of the ions to enable oscillation in the electron beam tobe set up. v

In carrying out the invention in one arrange ment, the focussing .coilis disposed relatively `close to the cathode, and the ratio yof theAdistance of the mid-.plane of the coil from lthe cathode. to theeffective diameter ,of the anode electrode is less than 1.5 to 1.k Afurther modification of the inventionapplied to acathode ray tube inwhich the anode is constituted bya silver or other conductive coatingjupon the interior wall `of the `cylindrical portion of the tubecontaining the electron gun, ,consists 4in .the division of the coatinginto two portions,fthat nearer the cathode of the tube being'maintainedvat a lower potential than thatv more remote from the cathode.

In order that the invention maybe more clearly understood and readilycarried into effect, two cathode ray tubes provided with electron beamcontrolling arrangements designed to operate in accordance with theinvention will now be described in greater detail by way of example withreference to Figures 1 and 2 of the accompanying drawing.

Figure l of the drawing shows part of a triode type of cathode ray tube4 of known construction employing an electromagnetic focussing coil 2and scanning coils 3. The cathode 4 is provided with a screen 5 and theanode 6 is constituted by a silver or other conductive coating extendingalong the interior Wall of the cylindrical portion of the tube from thecathode end to the flared portion 1. The focussing coil 2 normallyencircles the cylindrical portion of the tube and the scanning coils 3are situated between the focussing coil and the flared portion of thetube.

The dotted lines 8 represent the equipotential surfaces of theelectrostatic field set up by the cathode and cathode screen which aresubstantially at zero potential and owing to the distance between thecathode and the focussing coil this field is usually very weak in theregion of the focussing coil and consequently the ions present aresusceptible to the action of the magnetic field of the focussing coil.In applying the present invention to such a tube, the ratio of thedistance of the mid-plane of the coil from the cathode to the diameterof the anode is made smaller than in the known cases, with the resultthat the field of the coil is set up in a region in which theelectrostatic field is relatively strong. Preferably, the ratio of thedistance of the mid-plane of the focussing coil from the cathode to thediameter of the anode coating is less than 1.5:1. The mid-plane of thecoil is represented by the dotted lines 9. Such an arrangement enablesthe penetration of the eld due to the cathode screen to set up asuicient potential gradient near the mid-plane of the focussing coil tohinder the concentration of the positive ions by the magnetic field. Inorder to maintain the desired image magnification ratio the diameter ofthe portion of the tube between the focussing coil and the cathodescreen is made greater than would otherwise be the case.

In the cathode ray tube, a part of which is Y shown in Figure 2, the eldin the region of the focussing coil is increased by dividing the anodecoating into two sections I0 and II leaving a circular ring I 2 of clearglass at a position where the magnetic field due to the coil 2 isreasonably strong. `A difference of potential is maintained between thetwo sections of the anode, the section II nearest the fluorescent screenof the tube, not shown, being maintained at a higher potential than thesection nearer the cathode. Conveniently, assuming that the section IIof the anode nearest the fluorescent screen is maintained at a potentialof 3,000 or 4,000 volts by a.

source of supply I3 then the section I0 near the cathode 4 is maintainedat a potential which is between and 100 volts lower,` the difference ofpotential being produced by the source of supply I4. 'Ihe actualposition of the gap between the two sections of the anode is notcritical but it is preferred to arrange it at the end of the focussingcoil nearer the fluorescent screen of the tube as shown in the drawing.The desired effect may be achieved by maintaining the section of theanode nearer the fluorescent screen at a lower potential than thatnearer the cathode but the former method is preferred.

While particular values of voltages for application to the separatesections of the anode have been indicated, it will be understood that acertain amount of latitude is permissible but, on the other hand, thepotential difference between the sections of the anode should not besuicient to create an electrostatic lens of suiicient power to affectthe electron beam appreciably.

I claim:

1. A cathode ray device comprising an envelope having a cylindricalportion and a truncated conical portion, a cathode positioned within thecylindrical portion, and in the order named in register therewith acontrol electrode,- a first anode, a second anode immediately adjacentsaid first anode, the adjacent portions of said first and said secondanode having substantially identical physical dimensions, and a targetsurface, said rst electrode lying within said cylindrical portion ofsaid envelope, and said second anode lying both within the cylindricaland conical portion of said envelope, and a concentrated magneticfocused coil surrounding the portion of said first anode immediatelyadjacent the second anode.

2. A cathode ray device comprising an envelope having a cylindricalportion and a truncated conical portion, a cathode positioned within thecylindrical portion, and in the order named in register therewith acontrol electrode, a first anode, a second anode immediately adjacentsaid first anode, the adjacent portions of said first and said secondanode having substantially identical physical dimensions, both saidfirst and said second anodes being deposited upon the inner wall of saidenvelope, and a target surface, said first electrode lying within saidcylindrical portion of said envelope, and said second anode lying bothwithin the cylindrical and conical portion of said envelope, and aconcentrated magnetic focused coil surrounding the portion of said firstanode immediately adjacent the second anode.

LEONARD FRANCIS BROADWAY.

